1. Field of the Invention
The present invention relates to a method of calculation which estimates carrier mobility. In particular, the invention relates to a method of calculation which estimates the hole mobility and the electron mobility of a molecular film used for an organic electroluminescent element.
2. Description of the Related Art
In the evaluation of an organic electroluminescent element, the carrier mobility of an organic material is an important parameter that influences the characteristics of the element. The carrier mobility of an organic material can be estimated by fitting using a model formula of mobility in the space charge limited current region of a current-voltage (I-V) curve of a film formed of the material A Time of Flight (TOF) method is commonly used to directly measure the carrier mobility of organic materials which have insulating properties, and a lot of research has been conducted on carrier transportability.
However, because a film thickness of about 1 to 2 μm is necessary for measurements using the TOF method, when forming a film by vacuum evaporation or the like, a lot of material must be expended. Further, since a transient photocurrent curve cannot be seen clearly in measurements using the TOF method, it is also difficult to quantitatively estimate carrier mobility. In particular, it is difficult to quantitatively estimate the carrier mobility of a composite film formed from two or more materials.
Further, in the design of organic electroluminescent materials, besides estimates of electronic states such as the highest occupied molecular orbital (HOMO) level, the lowest unoccupied molecular orbital (LUMO) level, and the band gap by molecular orbital calculations, if estimates of hole mobility and electron mobility could be made, the design of materials that have charge transportability taken into consideration for each layer of an organic electroluminescent element would be possible.
Furthermore, for calculations concerning estimates of the electrical conduction properties of nanoscale devices, there is TranSIESTA-C. TranSIESTA-C is software which calculates voltage-current characteristics for two-probe systems under finite bias, based on density functional theory and non-equilibrium Green's function theory. In the two-probe systems, molecules are sandwiched between two electrodes. TranSIESTA-C can be applied to isolated molecular systems or to periodic systems such as crystals (for example, see Reference 1). However, this calculation software does not calculate the hole mobility or the electron mobility of an organic film, and from the standpoint of developing organic electroluminescent materials, the hole mobility and electron mobility of an organic film is important information.    [Reference 1] M. Brandbyge, J. L. Mozos, P. Ordejon, J. Taylor, K. Stokbro, Phys. Rev. B. 65, 165401 (2002)